With the growing application of filament-wound conductive fiber reinforced composites, particularly in aerospace, there is an increasing demand for nondestructive methods to detect and measure fiber anomalies such as service-incurred broken fibers and fabrication-related variations in fiber volume fraction or fiber density. Eddy current technology is ideally suited to these tasks since it is the conductive fibers which carry most of the current (carbon-carbons) or all of the current (graphite epoxies).
Eddy current transducers may be used alone or in various combinations. A particular transducer may be both the transmitter and the receiver if the transducer is operating in the reflection mode. When two or more are used, one or more may be a transmitter and have applied to it an alternating current or an alternating voltage. One or more of the others then functions as a receiver or pick-up coil. In this case the transmitter(s)/receiver(s) combination may function in the reflection mode, where both transmitter and receiver are on the same surface of the workpiece, or they may function in the through-transmission mode with the transmitter(s) on one surface and the receiver(s) on the opposite surface.
Common transducers are: (1) encircling coils which are wrapped around either the outer or the inner circumference of the workpiece; and (2) surface probes which generally are flat (pancake) coils used to inspect relatively flat surfaces. Surface probes may also have other forms, usually identified by the shape of the ferrite core placed within the coil. These include pot cores, E cores, and U cores.
The through transmission mode is typically used to inspect thin metal foils or sheets. Two pancake coils are mounted on the tines of a fork and the workpiece is passed between the coils. The combination of encircling coils on the outer and inner surfaces of a tube or cylinder are also used in the through-transmission mode. Encircling coils, in either mode, are typically used to inspect tubing having diameters on the order of inches or less. They are sensitive to cracks running in the axial direction and can locate a defect axially, but they provide no circumferential resolution. When large diameter tubes and cylinders are inspected by eddy current methods, surface probes are generally used for the axial and circumferential defect resolution they provide.
The inspection of filament-wound conducting fiber reinforced composites by eddy current methods poses a special problem. Structures of these materials typically have large diameters, on the order of feet rather than inches, and it is desirable to locate both the axial and circumferential position of detected defects. Both the structure size and the desired resolution suggest the use of surface probes which are used in the inspection of large diameter metal structures. However, surface probes require the presence in the workpiece of conducting paths which generally, if not exactly, image the current flowing in the eddy current coil. This requires that the test material possess conductivity in all directions in planes parallel to the surface. Yet filament-wound conductive fiber reinforced composites often provide conductive paths primarily in the circumferential direction--the general direction in which the conducting fibers are wound. Consequently, surface probes are ineffectual in the inspection of such structures. Encircling coils require conductivity only in the circumferential direction and so could be used with filament-wound conductive fiber reinforced composites. However, these coils provide no defect resolution in the circumferential direction. The invention addresses the problem posed by limited conduction paths in directions other than the circumferential and by the desire for circumferential resolution of defects.
Probe coils are effective in the eddy current inspection of large diameter metal structures. If back surface defects are to be detected, the eddy currents must fully penetrate the material under test. However, the diameter of the coil required to drive the field (and eddy currents) through the wall to the back surface of the workpiece is proportional to the thickness of the wall to be penetrated. On the other hand, large probes lead to poor spatial resolution in both axial and circumferential directions. Therefore conventional eddy current inspection systems cannot provide both high sensitivity and good resolution in thick sections.
An example of the current state of the art is Smith, U.S. Pat. No. 3,449,664, filed June 10, 1969. A transmitter/receiver combination is used to inspect conductive materials by eddy current methods. The transmitter is a pancake or probe coil and the receiver is a semiconductor device. The probe coil would not always induce eddy currents in filament-wound conductive fiber reinforced composites for reasons discussed above and so could not be used to replace the transmitter in the invention. The semiconducting device used as a receiver might work as well with the encircling coil transmitter as does the horseshoe receiver herein described, although different electronics would be required. This prior art suffers from the same shortcomings in the inspection of thick-wall metal structures as does any eddy current method utilizing probe coils.
Collins, et al., U.S. Pat. No. 4,747,310, filed May 31, 1988, and Collins, et al, U.S. Pat. No. 4,747,809, filed May 24, 1988, both provide probe coils for the eddy current inspection of composites. For the reasons discussed above, these probe coils would not always induce eddy current flow in filament-wound conductive fiber reinforced composites. In addition, the problems associated with thick-walled structures discussed above also apply.
Axisymmetric surface probe coils are useful in the eddy current inspection of semi-isotropic conductive fiber reinforced composite panels. For example, application Ser. No. 294,622 filed Jan. 9th, 1989 by Vernon et al., entitled "Method and Device for Measuring Resistivity" teaches methods and a device of eddy current inspection for estimating the electrical resistivity of materials. While the method applies to all materials with an approximate relative magnetic permeability of 1, it is particularly useful when the test material falls in one or more of the three categories: (1) the resistivity falls within a range for which there are no calibration standards; (2) only one surface of the test material is accessible; and (3) the resistivity is frequency dependent and must be determined over a particular range of frequencies. It is considered an advantage of this prior art invention to measure resistivity without electrical contact with the material under test, such as when the material is covered by a protective coating.
Another prior art reference filed by Vernon et al. on Jan. 9th, 1989 entitled "Method of Eddy Current Defect Depth Measurements", Ser. No. 294,621 is particularly useful for estimating the distance between the scanned surface and a subsurface defect of unknown geometry in electrically conductive materials. This device and its methods is particularly useful in measuring the depth of broken fiber damage such as may result from impact. Neither of these prior art applications, filed by Vernon et al., are capable of functioning on filament-wound conductive fiber reinforced composites because these materials generally lack sufficient conductivity in the axial direction. If they could be used for inspection of filament-wound structures, such as cylinders, cones and spheres, they would provide information on the axial and circumferential location of fiber anomalies, and through-thickness information about broken fiber damage. However, such probes require that a test material provide conductive paths in both the circumferential and axial directions. Filament-wound components, even when there are some fibers in the axial direction, do not always have sufficient conductivity in the axial direction to allow the use of axisymmetric surface probes.